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Concrete backgrounds aren't hard. Just cure it properly and rinse. No sealant needed, but you can use one if you want. And I will suggest if you are using a styrofoam base, make it in smaller pieces or find a way to brace them. Nothing worse than having it break when you are moving it, trust me.

Also you will need to glue it down depending on your styrofoam to concrete ratio. Without it you can have concrete suddenly come loose, flip over, and float around the tank to scratch everything up.